In an effort to make the world a healthier, cleaner, and safer place, there is interest in analyzing for chemicals of interest as it relates to industrial processes. For example, processed food items such as infant baby formula powder can be analyzed for lipid content or soil samples can be analyzed for toxic levels of a pesticide. These applications have created a need to analyze a large number of solid samples in an automated manner with a fast turn-around time.
In many instances, the chemical of interest must first be extracted from a solid sample so that it can be measured with an analytical technique. Solid liquid extraction is a method to solubilize an analyte from a solid sample. A solid sample can be any material or matrix that contains an analyte of interest. An analyte is a chemical that can be separated from a solid sample and subsequently quantitated using an analytical technique. Examples of an analyte may be an active ingredient in a drug tablet, a pesticide in a soil sample, or a lipid in a food item such as infant baby formula or corn chips.
In solid liquid extraction, the solid sample is treated with a liquid solvent to dissolve the analyte. A liquid solvent is selected that can solubilize the analyte, but not necessarily all of the other materials in the solid sample. Once the liquid solvent contains the dissolved analyte, it can be removed from any undissolved portion of the solid sample and then quantitated using a suitable analytical technique. An example of an analytical technique may be a liquid chromatograph coupled with a detector such as a conductivity detector, a charge detector, a UV-VIS spectrometer, or a mass-spectrometer.
In order to accelerate the solid liquid extraction process, the liquid solvent may be heated to enhance the solubility of the analyte. However, the temperature cannot be too high because the analyte can decompose or react with other chemicals in the solid sample. In addition, high temperature can cause the liquid solvent to vaporize and attenuate the solvating capability.
A supercritical fluid can be used to accelerate the solid liquid extraction process. A supercritical fluid is a substance at a temperature and a pressure above its critical point, and thus, develops properties similar to a liquid. An example of a supercritical fluid is carbon dioxide that is at a temperature and a pressure above 31.1° C. and 1070 pounds per square inch (PSI), respectively. While supercritical carbon dioxide is a good solvent for some applications that have low to moderate polarity analytes, it is less effective for extractions of high molecular weight and polar analytes. For example, environmental pollutants such as chlorinated dioxins from fly ash, polycyclic aromatic hydrocarbons, pesticides, semi volatile organic carbons, and nitroaromatics from urban air particulate matter have been reported (Harvey S, C W Wright, and B W Wright. 2001. Comprehensive Review of Applicable Supercritical Fluid Extraction Research. PNNL-13643, Pacific Northwest National Laboratory, Richland, W A; J. Chromatogr. A 703 (1995) 549-5; Journal of Hazardous Materials 161 (2009) 1-20; and U.S. Pat. No. 5,147,538, which are hereby fully incorporated by reference herein). For sample modifiers such as methanol or ethanol, usually in the <20 mole percent range, can be added to increase the polarity and have worked to some degree (Technology Watch, Volume 2, Issue 1, pp 1-12, July 2005; and Talanta 53 (2001) 771-782, which are hereby fully incorporated by reference herein). These modifiers do not work for all matrices, and thus, Applicants believe that there is a need for a method that allows extraction of species from a variety of matrices. It should be noted supercritical fluid liquid extraction requires relatively high pressures (typically much greater than about 100 PSI) making the instrumentation relatively expensive.
In solid liquid extraction, Applicants believe that there is a need to have a more efficient extraction process with a reduced amount of liquid solvent because proper clean-up and disposal of used liquid solvent can be expensive. At the same time, Applicants believe that this extraction process should be fast and use relatively inexpensive instrumentation that is amenable to automation.